One type of sensing system uses a permanent magnetic pole wheel. The magnetic pole wheel is typically used to detect the angular position and/or speed of a shaft to which the magnetic pole wheel is attached. A magnetic pole wheel includes a plurality of magnetic domains with alternating polarity (i.e., pole pairs) along the circumference of the magnetic pole wheel. These domains are magnetized in the axial, radial, and/or tangential direction of the magnetic pole wheel. The domains produce a magnetic field (B) as follows:B(ψ,r,z)=b(r,z)*sin(N*ψ+ψ0)  Equation I
where:                ψ is the azimuthal angle ranging from 0° to 360;        ψ0 is the phase shift;        r is the radial coordinate with r=0 on the axis of rotation;        z is the axial coordinate with z=0 in the mid-plane of the magnetic pole wheel;        N is the number of pole pairs along the circumference of the magnetic pole wheel; and        b(r,z) is a function describing the attenuation of the magnetic field versus distance of the magnetic pole wheel.        
Each pole pair includes one south pole and one north pole, which are the source and the sink of the magnetization. Two adjacent north poles are a distance (λ) apart, which is the magnetic pitch, with λ defined as:
                    λ        =                  perimeter          N                                    Equation        ⁢                                  ⁢        II            
where:                perimeter is the perimeter of the magnetic pole wheel.All three components of the magnetic field (i.e., the radial, the axial, and the azimuthal component) follow Equation I above, except for the value of ψ0, which denotes the phase shift.        
If the movement of a shaft is small, (e.g., if detection of the twisting of a shaft due to a moment load or any kind of deformation of a rigid body is desired), typical sensing systems are not sensitive enough to achieve a good accuracy and resolution. When a magnetic pole wheel rotates, the magnetic field pattern rotates synchronously. The magnetic field is detected by a magnetic field sensor. Typically, the magnetic field sensor generates a pulse if the magnetic field passes through zero, thereby digitizing the angular position of the magnetic pole wheel.
If a magnetic pole wheel has 10 pole pairs, the period of the magnetic field is 360°/10=36°. It is common for magnetic pole wheels to include 60 pole pairs for obtaining one pulse for every 6° of rotation. To increase the angular resolution of a magnetic pole wheel, more magnetic domains or pole pairs along the circumference of the magnetic pole wheel are used. The resolution of a magnetic pole wheel is limited, however, since it is impossible to magnetize domains that are too small.
In addition, the magnitude of the magnetic field decreases proportionally to exp(−2*π*z/λ) and exp(−2*π*r/λ). Therefore, at a constant air gap (i.e., the distance of the magnetic field sensor to the magnetic pole wheel), the magnetic field decreases substantially for small λ (i.e., for large N). Since there is typically some clearance between the magnetic pole wheel and the magnetic field sensor, the signal provided by the magnetic field sensor may decrease substantially over the lifetime of the sensing system due to growing slackness of bearings and/or deformation caused by loads.
Further, with typical magnetic pole wheels, the position of the magnetic pole wheel can be sensed only incrementally. That is, each time the magnetic pole wheel is turned by several degrees, the magnetic pole wheel provides a pulse. Therefore, the sensing system cannot detect the absolute position of the magnetic pole wheel; rather the sensing system merely detects changes in the position of the magnetic pole wheel. To determine the absolute position of the magnetic pole wheel, the pulses are summed. This summing of the pulses, however, is problematic if the magnetic pole wheel changes position while the magnetic field sensor is powered down. In this case, the sensing system looses track of the absolute position of the magnetic pole wheel.
Rotational position may also be detected by angle sensors that detect the direction of a homogeneous magnetic field acting on a magnetic field sensor including a sensor die. Typical sensing systems use a small permanent magnet that is attached to the end of a shaft and that is magnetized perpendicularly to the axis of rotation. The magnetic field of the permanent magnet is parallel to the sensor die if the sensor die is placed ahead of the permanent magnet and perpendicularly to the rotation axis. These sensing systems are fairly accurate (e.g., 1° accuracy), however, they are too slow to provide a real time signal with a delay on the order of microseconds as is needed for many applications. Also, the end of the shaft may not be available for attaching the permanent magnet. Another problem with these sensing systems is that they are susceptible to magnetic interference since the measurement is not differential. Any background magnetic field changes the direction of the working magnetic field from the permanent magnet and results in an angle error.
For these and other reasons, there is a need for the present invention.